Follow the Signal: Receptor, Neuron, Brain Region, Effector
Neural control questions ask where a structure sits, what it does, and how a signal travels. Learn the path of a reflex, the job of each brain region, the opposing effects of the two autonomic divisions, and the openings of the middle ear.
Nervous System: Neurons, Reflex Arc, Brain and Ear is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Neurons and the reflex arc
Nissl granules are clusters of rough endoplasmic reticulum and free ribosomes found in the cell body and dendrites of neurons, where they make proteins; they are not found in muscle, bone or cartilage cells. A reflex arc is the shortest path from stimulus to response: a receptor (sensory) neuron carries the impulse to the central nervous system, one or more interneurons (internuncial neurons) connect it, and an effector (motor) neuron carries the command to the effector, a muscle or gland. Most reflex arcs therefore have one receptor neuron, one or more interneurons and one effector neuron.
Brain regions and what they control
The cerebrum is the largest part of the brain and its two hemispheres are joined by the corpus callosum, a thick whitish band of fibres (the largest white-matter structure), which carries information between the sides of the brain; it should not be confused with corpus luteum or corpus albicans, which are ovarian structures, or corpus striatum, a deeper structure of the basal ganglia. The hypothalamus maintains homeostasis through the autonomic system and hormones, and controls hunger, thirst and satiety. The cerebellum coordinates movement and balance, and is the region involved in Chiari malformation. The pons helps regulate sleep and relays signals; its pneumotaxic centre works with the medulla to set the rhythm of breathing.
Sympathetic and parasympathetic effects
The two autonomic divisions usually have opposite effects. The sympathetic division prepares the body for action: salivation decreases, pupils dilate, bronchi dilate, the heart speeds up and adrenaline is released, mobilising glucose from glycogen. The parasympathetic division favours rest and digestion: pupils constrict, bronchi constrict, salivation and gastric activity increase, the bladder contracts and energy is stored, for example glucose converted to glycogen. To check an option, test each half against its division; options that swap the two divisions are wrong.
The middle ear openings
The tympanic cavity (middle ear) is connected with the inner ear (auditory capsule) by two small membrane-covered openings: the fenestra ovalis (oval window), where the stapes passes vibrations in, and the fenestra rotunda (round window), which relieves the pressure of the fluid. The cavity is connected to the pharynx by the Eustachian tube.
OlympIQ exam lens
- ✓Nissl granules (rough ER and ribosomes) occur in neuron cell bodies and dendrites
- ✓Reflex arc: receptor neuron, one or more interneurons in the CNS, effector neuron
- ✓Corpus callosum joins the cerebral hemispheres; hypothalamus = homeostasis; cerebellum = coordination; pons = pneumotaxic centre and sleep
- ✓Sympathetic: salivation down, adrenaline up, pupils and bronchi dilate; parasympathetic: opposite, bladder contracts, glucose stored as glycogen
- ✓Middle ear to inner ear: fenestra ovalis and fenestra rotunda
Do not mix up the autonomic divisions when an option lists two effects. Each half of the option names an effect of one division, so check the first half against the sympathetic division and the second half against the parasympathetic division.
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